The invention relates to the production of an electrode, the most important components of which consist of a noble metal catalyst and a proton-conducting polymer. Such electrodes are used, inter alia, in fuel cells which contain a proton-conducting polymer membrane as electrolyte (SPFC, Solid Polymer Fuel Cell). A fuel cell of this type is able to convert chemical energy into electrical energy and heat in a clean, quiet and efficient manner. Possible applications are, inter alia, electric transport, heat/power generation on a scale of 1–250 kW and portable equipment.
Such a fuel cell has two electrodes, an anode and a cathode, at which, respectively, a fuel is oxidised and oxidant is reduced. The fuel used can be hydrogen, a hydrogen-containing gas or an organic compound, for example methanol. The oxidant used is usually atmospheric oxygen.
The optimum operating temperature of a low-temperature fuel cell based on a proton-conducting polymer is around 60–80° C. The majority of active electrodes for the oxidation of hydrogen and the reduction of oxygen at such temperatures and in an acid medium contain platinum as catalytically active material. Hydrogen-containing gases which are produced by the reaction of a hydrocarbon in a so-called reformer also contain, inter alia, carbon dioxide and carbon monoxide in addition to hydrogen. Carbon monoxide in particular has a highly adverse effect on the activity of platinum for the electrochemical conversion of hydrogen to protons. A catalyst that contains a mixture of platinum and a second metal, for example ruthenium or molybdenum, in general has a higher activity for electrochemical oxidation of hydrogen in carbon monoxide-containing gases than catalysts based on platinum. With regard to the reduction of oxygen, it is known that catalysts consisting of a mixture of platinum and a second metal, for example chromium or nickel, can have a higher activity than catalysts based on platinum alone.
For efficient utilisation of expensive noble metals in fuel cell electrodes it is extremely important that the surface area/mass ratio of the noble metal used is as high as possible. This is achieved by applying the noble metal from a solution to a support material in such a controlled manner that the crystallite diameter is approximately 2–4 nm. The support material used is generally carbon because of the requisite electrical conductivity. By making use of a carbon with a high surface area per unit mass it is possible to apply an appreciable quantity of noble metal per unit volume of carbon. Widely used support materials are Vulcan XC-72, a carbon powder with a BET surface area of approximately 250 m2/g, Shawinigan Acetylene Black, a carbon powder with a BET surface area of approximately 80 m2/g, and Black Pearls, a carbon powder with a BET surface area of approximately 1475 m2/g.
The requirement for a high electrochemical rate of reaction per unit catalytic surface area is that the catalytic surface area is readily accessible to the gaseous reactants, and to protons and electrons. In addition, in the case of the oxygen reduction reaction it must be possible efficiently to discharge the water produced in order thus to keep the accessibility to oxygen high. For good accessibility to gaseous reactants, the electrode must have a certain porosity, which in SPFCs which function well is of the order of 50%. To achieve a sufficiently high proton conductivity use is usually made of electrodes which, in addition to platinum on carbon, also contain the same proton-conducting polymer as that used to produce the electrolytic membrane. The percentage of proton-conducting polymer must not be too high, since the electronic conductivity and the gas accessibility decrease as the content of proton-conducting polymer increases. In general, a concentration of proton-conducting polymer of 10–50%, in particular 20–30%, based on dry weight, is suitable.
An SPFC electrode consists roughly of two different layers: a thin catalytic layer approximately 5–20 μm thick, where the actual electrochemical reaction takes place, and a thick porous layer approximately 100–300 μm thick, which is termed the electrode backing. The function of this thick layer is to distribute the gas to electrode sections which are not opposite a gas channel, to guide electrons in the lateral direction and to ensure effective water transport from and to the electrode.
The catalytic layer can be applied either to the electrode backing or to the electrolytic membrane. Various techniques for application are known, including atomising, screen printing and coating. In order to make use of these techniques the noble metal-containing carbon particles and the proton-conducting polymer must have been dispersed in a suitable solvent. This dispersion is termed ink. The entire dispersion must have a rheology which makes it possible to process the ink in the manufacturing equipment used. In addition, the solvents used must evaporate within a practical timescale. Evaporation that is too rapid leads to a changing rheology during electrode production, with the consequence that the production of electrodes is not reproducible. In addition, evaporation that is too rapid leads to agglomeration of solid ink components, as a result of which the production process is interrupted. However, it must be possible to remove the solvents used at a temperature of at most 150° C. at a reasonable speed, within at most one hour. Above this temperature of 150° C. changes take place in the proton-conducting polymer in the electrode, as a result of which proton conductivity in the electrode decreases.
In order to obtain a well-dispersed electrode ink use is often made of additives such as binders and surfactants. The function of a surfactant is to reduce any repulsions between the surface of the dispersed particles and the dispersing medium so as thus to obtain a stable dispersion. A binder is in general a component that has the effect of increasing the viscosity.
Examples of components which have the effect of increasing the viscosity are carboxymethylcellulose, polyethelene glycol, polyvinyl alcohol, polyvinylpyrrolidone and other polymer compounds. As a consequence of the polymer character of such compounds which increase the viscosity, these compounds form part not only of the electrode ink but also of the final electrode. Not only is this component then an electrode constituent that has no function in the final electrode but, by interaction with the noble metal surface of the active phase, such a component can also have an adverse effect on the electrochemical activity of the electrode. This results in a reduced capacity per unit of electrode surface.
U.S. Pat. No. 5,330,860 in the name of W. Grot et al. teaches that the proton-conducting, perfluorinated sulphonic acid polymer, such as Nafion, required for the electrode can serve as binder in the electrode ink. Addition of a supplementary component that increases the viscosity becomes superfluous as a result. According to the cited patent, the solvent used is an ether, preferably 1-methoxy-2-propanol. However, such a solvent has too high a vapour tension at room temperature, specifically 12 mbar, as a result of which the viscosity of the electrode ink is subject to change during the electrode production process. Such an ether compound also has adverse consequences for health.
An attractive alternative to the use of a hydrocarbon such as 1-methoxy-2-propanol is water. The use of water as solvent in an electrode ink is described in U.S. Pat. No. 5,716,437 in the name of Denton et al. Water has no effect whatsoever on health and, if suitable, would be the ideal solvent for the production of electrodes. However, water has too high a vapour tension at room temperature, specifically 17 mbar. As a consequence the viscosity of the electrode ink changes during the production process. In addition it is very difficult to print hydrophobic surfaces, which include the electrode backing surfaces which are most common for use in an SPFC, with a water-based ink.
An electrode ink which consists of two immiscible components is described in EP-A 0 945 910. One of the components is an ink which contains the catalyst with the conducting polymer (ionomer) in a polar solvent such as an alcohol or diol, for example propylene glycol, dipropylene glycol, glycerol or hexylene glycol. The other component is an ink containing catalyst without ionomer in an apolar solvent, such as fatty acid esters, for example methyl dodecanoate. After combining the two inks, an electrode having an inhomogeneous microstructure is produced, the inhomogeneity serving to improve the gas transport in the catalytic layer and thus to increase the capacity of the fuel cell. However, the method according to EP-A 0 945 910 is laborious and, moreover, the electrode performance is not yet completely satisfactory.
A method for electrode production in which the starting material used is a colloidal solution of the polymer is described by M. Uchida et al., “New Preparation Method for Polymer-Electrolyte Fuel Cells”, J. Electrochem. Soc. 142 (1995), 463–468. Propanediol is regarded as an unsuitable solvent by Uchida et al. because it is not possible to form polymer colloids therein because the dielectric constant of propanediol is too high.